Plant combination for producing steel and method for operating the plant combination

ABSTRACT

The invention relates to a plant complex for steel production comprising a blast furnace for producing pig iron, a converter steel mill for producing crude steel, a gas-conducting system for gases that occur in the production of pig iron and/or in the production of crude steel, and a power-generating plant for electricity generation. The power-generating plant is operated with a gas that comprises at least a partial amount of the blast-furnace top gas that occurs in the production of pig iron and/or a partial amount of the converter gas. According to the invention, a chemical or biotechnological plant is provided and connected to the gas-conducting system and arranged in parallel with the power-generating plant with respect to the gas supply. Externally obtained electricity and power-generating plant electricity are used to cover the electricity demand of the plant complex.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national phase of, and claims priority to,International Patent Application No. PCT/EP2014/003320, filed Dec. 11,2014, which designated the U.S. and which claims priority to GermanPatent Application Number DE 10 2013 113 913.2, filed Dec. 12, 2013.These applications are each incorporated by reference herein in theirentireties.

BACKGROUND

1. Field of the Invention

The invention relates to a plant complex for steel production and to amethod for operating the plant complex.

2. Description of the Related Art

Pig iron is obtained in a blast furnace from iron ores, additives suchas coke and other reducing agents such as coal, oil, gas, biomasses,recycled waste plastics or other substances containing carbon and/orhydrogen. CO, CO₂, hydrogen and water vapour inevitably occur asproducts of the reduction reactions. Apart from the aforementionedconstituents, a blast-furnace furnace top gas drawn off from theblast-furnace process often has a high content of nitrogen. The amountof gas and the composition of the blast-furnace top gas are dependent onthe feedstock and the operating mode and are subject to fluctuations.Typically, however, blast-furnace top gas contains 35 to 60% by volumeN₂, 20 to 30% by volume CO, 20 to 30% by volume CO₂ and 2 to 15% byvolume H₂. Around 30 to 40% of the blast-furnace top gas produced in theproduction of the pig iron is generally used for heating up the hot airfor the blast-furnace process in air heaters; the remaining amount oftop gas may also be used externally in other areas of the works forheating purposes or for electricity generation.

In the converter steel mill, which is arranged downstream of theblast-furnace process, pig iron is converted into crude steel. Byblowing oxygen onto liquid pig iron, troublesome impurities such ascarbon, silicon, sulphur and phosphorus are removed. Since the oxidationprocesses cause an intense development of heat, scrap is often added inamounts of up to 25% with respect to the pig iron as a coolant.Furthermore, lime is added for forming slag and an alloying agent. Aconverter gas that has a high content of CO and also contains nitrogen,hydrogen and CO₂ is drawn off from the steel converter. A typicalconverter gas composition has 50 to 70% by volume CO, 10 to 20% byvolume N₂, about 15% by volume CO₂ and about 2% by volume H₂. Theconverter gas is either burned off or, in the case of modern steelmills, captured and passed on to be used for providing energy.

The plant complex may optionally be operated in combination with acoking plant. In this case, the plant complex described at the beginningadditionally comprises a coke-oven plant, in which coal is convertedinto coke by a coking process. In the coking of coal into coke, acoke-oven gas occurs, containing high hydrogen content and considerableamounts of CH₄. Typically, coke-oven gas contains 55 to 70% by volumeH₂, 20 to 30% by volume CH₄, 5 to 10% by volume N₂ and 5 to 10% byvolume CO. In addition, the coke-oven gas has fractions of CO₂, NH₃ andH₂S. In practice, the coke-oven gas is used in various areas of theworks for heating purposes and in the power-generating process forelectricity generation. In addition, it is known to use coke-oven gastogether with blast-furnace top gas or with converter gas for producingsyngases. According to a method known from WO 2010/136313 A1, coke-ovengas is separated into a hydrogen-rich gas stream and a residual gasstream containing CH₄ and CO, the residual gas stream being fed to theblast-furnace process and the hydrogen-rich gas stream being mixed withblast-furnace top gas and processed further into a syngas. It is knownfrom EP 0 200 880 A2 to mix converter gas and coke-oven gas and use themas a syngas for methanol synthesis.

In an integrated metallurgical plant that is operated in combinationwith a coking plant, approximately 40 to 50% of the raw gases that occuras blast-furnace top gas, converter gas and coke-oven gas are used forchemical engineering processes. Approximately 50 to 60% of the gasesproduced are fed to the power-generating plant and used for electricitygeneration. The electricity produced in the power-generating plantcovers the electricity demand for the production of pig iron and crudesteel. Ideally, the energy balance is closed, so that, apart from ironores and carbon in the form of coal and coke as sources of energy, nofurther energy input is necessary and, apart from crude steel and slag,no product leaves the plant complex.

SUMMARY

One object of the invention includes further improving thecost-effectiveness of the overall process and providing a plant complexwith which it is possible to reduce the costs for steel production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified block diagram of a plant complex for producingsteel comprising a blast furnace for producing pig iron and a convertersteel mill for producing crude steel, a power-generating plant and achemical or biotechnological plant.

FIG. 2 shows a simplified block diagram of a plant complex whichcomprises in addition to a blast furnace for producing pig iron and aconverter steel mill for producing crude steel, a power-generating plantand a chemical or biotechnological plant also a coke-oven plant.

FIG. 3 shows the block diagram of a plant complex corresponding to FIG.2 with an additional plant for producing hydrogen.

DETAILED DESCRIPTION

According to one embodiment of the invention, a plant complex for steelproduction comprises a blast furnace for producing pig iron, a convertersteel mill for producing crude steel, a gas-conducting system for gasesthat occur in the production of pig iron and/or in the production ofcrude steel, and also a power-generating plant for electricitygeneration. The power-generating plant is designed as a gas-turbinepower-generating plant or gas-turbine and steam-turbine power-generatingplant that is operated with a gas that comprises at least a partialamount of the blast-furnace top gas that occurs in the production of pigiron in the blast furnace and/or a partial amount of the converter gasthat occurs in the converter steel works.

Proceeding from a plant complex for producing steel comprising a blastfurnace for producing pig iron, a converter steel mill for producingcrude steel, a gas-conducting system for gases that occur in theproduction of pig iron and/or in the production of crude steel, and apower-generating plant for electricity generation, according to theinvention a chemical or biotechnological plant is provided, connected tothe gas-conducting system and arranged in parallel with thepower-generating plant with respect to the gas supply. According to theinvention, the gas-conducting system comprises an operationallycontrollable gas diverter for dividing the streams of gas that are fedto the power-generating plant and the chemical or biotechnologicalplant. Also the subject of the invention is a method for operating aplant complex that has a blast furnace for producing pig iron, aconverter steel mill, a power-generating plant and a chemical plant orbiotechnological plant. According to the method according to oneembodiment of the invention, at least a partial amount of theblast-furnace top gas that occurs in the production of pig iron in theblast furnace and/or a partial amount of the converter gas that occursin the production of crude steel is used as a useful gas for operatingthe power-generating plant and the chemical plant or biotechnologicalplant. Externally obtained electricity and power-generating plantelectricity, which is produced by the power-generating plant of theplant complex, is used to cover the electricity demand of the plantcomplex. This involves establishing the proportion of electricityaccounted for by the externally obtained electricity with respect to theoverall electricity demand of the plant complex as a variable processparameter and establishing the amount of useful gas fed to thepower-generating process in dependence on this process parameter. Thepart of the useful gas that is not used for electricity generation isused after a gas-conditioning operation as a syngas for producingchemical products or is fed after a gas-conditioning operation to abiotechnological plant and used for biochemical processes.

In the chemical plant, chemical products can be produced from syngasesthat respectively contain the components of the end product. Chemicalproducts may be for example ammonia or methanol or else otherhydrocarbon compounds.

For producing ammonia, a syngas that contains nitrogen and hydrogen inthe correct ratio must be provided. The nitrogen can be obtained fromblast-furnace top gas. Blast-furnace top gas or converter gas may beused as the hydrogen source, hydrogen being produced by conversion ofthe CO fraction by a water-gas-shift reaction (CO+H₂O

CO₂+H₂). For producing hydrocarbon compounds, for example methanol, itis necessary to provide a syngas consisting substantially of CO and/orCO₂ and H₂ that contains the components carbon monoxide and/or carbondioxide and hydrogen in the correct ratio. The ratio is often describedby the module (H₂−CO₂)/(CO+CO₂). The hydrogen may be produced forexample by conversion of the CO fraction in the blast-furnace top gas bya water-gas-shift reaction. Converter gas may be used for providing CO.Blast-furnace top gas and/or converter gas may serve as a source of CO₂.

Within the scope of the invention, a biotechnological plant may also beused instead of a chemical plant for producing products from syngas. Theplant concerned is a plant for the fermentation of syngas. The syngas isused biochemically by way of a fermentation process, it being possibleto produce products such as alcohols (ethanol, butanol), acetone ororganic acids. These products, which are produced by fermentation ofsyngas, are also only mentioned by way of example in the present case.

According to a preferred embodiment of the invention, the plant complexadditionally comprises a coke-oven plant. If the production of pig ironand the production of crude steel are operated in combination with acoking plant, a partial amount of the blast-furnace top gas that occursin the production of pig iron and/or a partial amount of the convertergas that occurs in the converter steel mill may be mixed with a partialamount of the coke-oven gas that occurs in the coke-oven plant and themixed gas may be used as a useful gas. A mixture of coke-oven gas andblast-furnace top gas or a mixed gas comprising coke-oven gas, convertergas and blast-furnace top gas may be used for producing a syngas, forexample for ammonia synthesis. A mixed gas comprising coke-oven gas andconverter gas or a mixed gas comprising coke-oven gas, converter gas andblast-furnace top gas is suitable for producing hydrocarbon compounds.The described chemical products that can be produced in a chemical plantfrom blast-furnace top gas, converter gas and coke-oven gas are onlyapplication examples for explaining the variants of the method that aredescribed herein.

The raw gases—coke-oven gas, converter gas and blast-furnace top gas—maybe conditioned individually or in combinations as a mixed gas and thenfed to the chemical plant as syngases. The conditioning of coke-oven gasin particular comprises a cleaning of the gas to separate outtroublesome contents, in particular tar, sulphur and sulphur compounds,aromatic hydrocarbons (BTX) and high-boiling hydrocarbons. Agas-conditioning operation is also necessary for producing the syngas.In the course of the gas conditioning, the proportion of the componentsCO, CO₂ and H₂ within the raw gas is changed. The gas conditioningcomprises for example pressure swing adsorption for separating out andenriching H₂ and/or a water-gas-shift reaction for converting CO intohydrogen and/or a steam reformer for converting the CH₄ fraction into COand hydrogen in the coke-oven gas.

In the case of the method according to the invention, at least a partialamount of the blast-furnace top gas that occurs in the production of pigiron in the blast furnace and/or a partial amount of the converter gasthat occurs in the converter steel mill is used as raw gas, in order toproduce products, that is to say substances of value, from them bychemical reactions in a chemical plant or by biochemical processes in abiotechnological plant. According to a preferred embodiment of theinvention, the plant is operated in combination with a coking plant andcoke-oven gas is integrated in the use. As a consequence of using partof these gases, the plant complex has a deficit of electricity, whichhas to be obtained externally. The externally obtained electricity mayoriginate from conventional power-generating plants or be obtained fromrenewable energy sources. Preferably, the externally obtainedelectricity is obtained completely or at least partially from renewableenergy and originates for example from wind turbine generator plants,solar plants, geothermal power-generating plants, hydroelectricpower-generating plants, tidal power-generating plants and the like. Toachieve operation of the plant complex that is as cost-effective aspossible, at times of low electricity prices, electricity is bought inand used for supplying to the plant complex, and the part of the usefulgas that is not used for electricity generation is used for producingchemical products after a gas-conditioning operation in a chemical plantand/or a biotechnological plant. At times of high electricity prices, onthe other hand, the useful gas is completely or at least mostly fed tothe power-generating plant in order to produce electricity for supplyingto the plant complex. The chemical plant or biotechnological plant iscorrespondingly operated at a lower output at times of high electricityprices. A closed-loop control system is provided for operating themethod, establishing the alternating operation of the power-generatingplant on the one hand and the chemical plant or biotechnological planton the other hand in dependence on a variable process parameter. Theprocess parameter is preferably determined in dependence on a functionthat includes the price for the externally obtained electricity and thecosts for producing the power-generating plant electricity as variables.

The method according to the invention makes it possible for the plantcomplex to be operated cost-effectively. The method according to theinvention thereby also makes use in particular of the fact that theefficiency of a power-generating process for producing electricity isworse than the efficiency of a chemical plant or a biotechnologicalplant in which chemical products are produced by chemical reactions orby biochemical processes from syngas.

The power output of the power-generating plant can be controlled between20% and 100%, in dependence on the amount of useful gas fed to thepower-generating process. A gas-turbine power-generating plant orgas-turbine and steam-turbine power-generating plant is preferably usedas the power-generating plant.

The power output of the chemical plant or of the biotechnological plantis controlled in dependence on the amount of mixed gas fed to theseplants. A major challenge for the chemical plant is that of finding away of operating dynamically with changing plant loads. The way ofoperating with changing plant loads can be realized in particular by thechemical plant having a plurality of small units arranged in parallel,which are individually switched on or off depending on the availablestream of useful gas.

The use of a biotechnological plant has the advantage that abiotechnological plant is more flexible with respect to load changesthan a chemical plant.

The plant complex for steel production that is represented in FIG. 1comprises a blast furnace 1 for producing pig iron, a converter steelmill 2 for producing crude steel, a power-generating plant 3 forproducing electricity and a chemical or biotechnological plant 11.

In the blast furnace 1, pig iron 6 is obtained substantially from ironore 4 and reducing agents 5, in particular coke and coal. Reductionreactions cause the production of a blast-furnace top gas 7, whichcontains nitrogen, CO, CO₂ and H₂ as the main constituents. In theconverter steel mill 2 that is arranged downstream of the blast-furnaceprocess, pig iron 6 is converted into crude steel 8. By blowing oxygenonto the liquid pig iron, troublesome impurities, in particular carbon,silicon and phosphorus, are removed. For cooling, scrap may be added inamounts of up to 25% with respect to the amount of pig iron.Furthermore, lime is added for forming slag and an alloying agent. Atthe top of the converter, a converter gas 9 that has a very highproportion of CO is drawn off.

The power-generating plant 3 is designed as a gas-turbinepower-generating plant or gas-turbine and steam-turbine power-generatingplant and is operated with a gas that comprises at least a partialamount of the blast-furnace top gas 7 that occurs in the production ofpig iron in the blast furnace 1 and a partial amount of the convertergas 9 that occurs in the converter steel works 2. A gas-conductingsystem is provided for carrying the gases.

According to the overall balance represented in FIG. 1, carbon is fed tothe plant complex as a reducing agent 5 in the form of coal and coke andalso iron ore 4. Occurring as products are crude steel 8 and raw gases7, 9, which differ in amount, composition, calorific value and purityand are used again at various points in the plant complex. In an overallconsideration, 40 to 50%, usually approximately 45%, of the raw gases 7,9 are returned again into the metallurgical process for producing pigiron or producing crude steel. Between 50 and 60%, usually approximately55%, of the raw gases 7, 9 can be used for operating thepower-generating plant 3. The power-generating plant 3 operated with amixed gas 10 comprising blast-furnace top gas 7 and converter gas 9 isdesigned in such a way that it can cover the electricity demand of theplant complex.

According to the representation in FIG. 1, a chemical orbiotechnological plant 11 is provided, connected to the gas-conductingsystem and arranged in parallel with the power-generating plant 3 withrespect to the gas supply. The gas-conducting system has anoperationally controllable gas diverter 12 for dividing the streams ofgas that are fed to the power-generating plant 3 and the chemical orbiotechnological plant 11. Provided upstream of the gas diverter in thedirection of flow is a mixing device 13, for producing the mixed gas 10consisting of blast-furnace top gas 7 and converter gas 9.

In the case of the plant complex represented in FIG. 1, at least apartial amount of the blast-furnace top gas 7 that occurs in theproduction of pig iron in the blast furnace 1 and a partial amount ofthe converter gas 9 that occurs in the production of crude steel areused as a useful gas for operating the power-generating plant 3 and thechemical or biotechnological plant 11. Externally obtained electricity14 and power-generating plant electricity 15, which is produced by thepower-generating plant 3 of the plant complex, are used to cover theelectricity demand of the plant complex. The proportion of electricityaccounted for by the externally obtained electricity 14 with respect tothe overall electricity demand of the plant complex is established as avariable process parameter and the amount of useful gas N1 fed to thepower-generating plant 3 is determined in dependence on this processparameter. The part of the useful gas N2 that is not used forelectricity generation is used after a gas-conditioning operation as asyngas for producing chemical products 16 or is fed after agas-conditioning operation to the biotechnological plant and used forbiochemical processes.

The externally obtained electricity 14 is preferably obtained completelyor at least partially from renewable energy and originates for examplefrom wind turbine generator plants, solar plants, hydroelectricpower-generating plants and the like. The process parameter on the basisof which the amount of useful gas N1 that is fed to the power-generatingprocess is established is determined in dependence on a function thatincludes the price for the externally obtained electricity and the costsfor producing the power-generating plant electricity 15 as variables. Toachieve operation of the plant complex that is as cost-effective aspossible, at times of low electricity prices, electricity is brought inas external electricity 14 and used for supplying electricity to theplant complex, the part of the useful gas N2 that is not used forproducing electricity being fed to the chemical or biotechnologicalplant 11 and used for producing chemical products 16 after agas-conditioning operation. At times of high electricity prices, the rawgases 7, 9 that occur in the production of pig iron and the productionof crude steel are fed to the power-generating plant 3 in order toproduce electricity for supplying to the plant complex. The chemicalplant 11 or the alternatively provided biotechnological plant iscorrespondingly operated at a lower output at times of high electricityprices.

The power output of the power-generating plant 3 is controlled between20% and 100%, in dependence on the amount of useful gas N1 fed to thepower-generating process. The power output of the chemical plant 11 orof the biotechnological plant is controlled in dependence on the amountof useful gas N2 fed to this plant. A major challenge for the chemicalplant 11 is that of finding a way of operating dynamically with changingloads. This can be realized by the chemical plant 11 having a pluralityof small units arranged in parallel, which are individually switched onor off depending on the available amount of useful gas N2.

In the exemplary embodiment of FIG. 2, the plant complex additionallycomprises a coke-oven plant 17. In the coking of coal 18 into coke 19,coke-oven gas 20 occurs, containing a high proportion of hydrogen andCH₄. Parts of the coke-oven gas 20 may be used for the heating of theair heaters in the blast furnace 1. The gas-conducting system includes agas distribution for the coke-oven gas 20. Provided upstream of the gasdiverter 12 in the direction of flow is a mixing device 13, forproducing a mixed gas 10 consisting of blast-furnace top gas 7,converter gas 9 and coke-oven gas 20. With the gas diverter 12, thestreams of gas that are fed to the power-generating plant 3 and thechemical or biotechnological plant 11 can be controlled.

During the operation of the plant represented in FIG. 2, a partialamount of the blast-furnace top gas 7 that occurs in the production ofpig iron and/or a partial amount of the converter gas 9 that occurs inthe converter steel mill are mixed with a partial amount of thecoke-oven gas 20 that occurs in the coke-oven plant 17. The mixed gas 10is used as a useful gas for operating the power-generating plant 3 andthe chemical plant 11 or biotechnological plant.

The blast-furnace top gas 7, the converter gas 9 and the coke-oven gas20 may be combined with one another in any way desired. The combinationof gas streams 7, 9, 20 depends on the desired syngas or the productthat is to be produced in the chemical plant 11 or the biotechnologicalplant by using the syngas.

For example, it is possible within the scope of the invention thatblast-furnace top gas 7 and converter gas 9 are mixed, that a syngas isproduced from the mixed gas after a gas-conditioning operation and thatconditioned coke-oven gas 20 is additionally admixed with the syngas orthe cleaned mixed gas before the further processing to form the syngas.

Furthermore, there is the possibility that a syngas is produced fromblast-furnace top gas 7 after a gas-conditioning operation and thatconditioned coke-oven gas 20 is additionally admixed with the syngas orthe cleaned blast-furnace top gas before the further processing to formthe syngas.

Finally, there is the possibility that a syngas is produced fromconverter gas 9 after a gas-conditioning operation and that conditionedcoke-oven gas 20 is additionally admixed with the syngas or the cleanedconverter gas before the further processing to form the syngas.

In the case of the operating mode represented in FIGS. 1 and 2, thecarbon content and the nitrogen content of the raw gases that occurduring the operation of the plant complex cannot be used completely forproducing chemical products, since there is a hydrogen deficit. In orderto use the carbon content and the nitrogen content of the useful gascompletely for the production of chemical substances of value, the plantcomplex represented in FIG. 3 additionally has a plant 21 for producinghydrogen, which is connected to the gas-conducting system by ahydrogen-carrying line 22. The plant 21 for producing hydrogen may be inparticular an electrolysis plant for the electrolysis of water.Electrolysis of water is energy-intensive to operate and is thereforeprimarily put into operation at times of low electricity prices, atwhich the chemical plant 11 or biotechnological plant is also operatedand the power-generating plant 3 is operated at a lower output. Thehydrogen that is additionally produced is fed to the chemical plant 11together with the mixed gas. This allows the capacity of the chemicalplant 11 to be increased significantly. The same applies correspondinglyif a biotechnological plant is provided instead of the chemical plant11.

1.-18. (canceled)
 19. A plant complex for the production of steel,comprising: a blast furnace for producing pig iron; a converter steelmill for producing crude steel; a gas-conducting system for gases thatoccur in the production of pig iron and/or the production of crudesteel; a power-generating plant for electricity generation; and achemical or biotechnology plant; wherein: the power-generating plant isdesigned as a gas-turbine power-generating plant or gas-turbine andsteam-turbine power-generating plant′ the power-generating plant isoperated with a gas that comprises at least a partial amount ofblast-furnace top gas that occurs in the production of pig iron in theblast furnace and/or a partial amount of the converter gas that occursin the converter steel mill; the chemical or biotechnological plant isconnected to the gas-conducting system and arranged in parallel with thepower-generating plant (3) with respect to the gas supply; and thegas-conducting system comprises an operationally controllable gasdiverter for dividing the streams of gas that are fed to thepower-generating plant and the chemical or biotechnological plant. 20.The plant complex according to claim 19, wherein the plant complexadditionally comprises a coke-oven plant, and wherein the gas-conductingsystem includes a gas distribution for coke-oven gas that occurs in acoking process in the coke-oven plant.
 21. The plant complex accordingto claim 19, wherein the gas-conducting system includes, upstream of thegas diverter in the direction of flow, a mixing device for producing amixed gas comprising at least one of blast-furnace top gas, convertergas, and coke-oven gas, and wherein the streams of gas that are fed tothe power-generating plant and the chemical or biotechnological plantcan be controlled by means of the gas diverter.
 22. The plant complexaccording to one of claim 19, wherein the plant complex additionally hasa plant for producing hydrogen, which is connected to the gas-conductingsystem by a hydrogen-carrying line.
 23. The plant complex according toclaim 22, wherein the plant for producing hydrogen is an electrolysisplant for the electrolysis of water.
 24. A method for operating a plantcomplex comprising a blast furnace for producing pig iron, a convertersteel mill, a power-generating plant, and a chemical plant orbiotechnological plant, the method comprising: a) using at least one ofa partial amount of the blast-furnace top gas that occurs in theproduction of pig iron in the blast furnace and a partial amount of theconverter gas that occurs in the production of crude steel as a usefulgas for operating the power-generating plant and the chemical plant orbiotechnological plant; b) using externally obtained electricity andpower-generating plant electricity, which is produced by thepower-generating plant of the plant complex, to cover the electricitydemand of the plant complex; c) determining a variable processparameter, wherein the process parameter is dependent on the proportionof electricity accounted for by the externally obtained electricity withrespect to the overall electricity demand of the plant complex; d)determining the amount of useful gas fed to the power-generating processbased on the process parameter determined in step (c); and e) using thepart of the useful gas that is not used for electricity generation,after a gas-conditioning operation: e1) as a syngas for producingchemical products; or e2) by the biotechnological plant for biochemicalprocesses, wherein the part of the useful gas is fed to thebiotechnological plant.
 25. The method according to claim 24, wherein:the plant complex additionally comprises a coke-oven plant; at least oneof a partial amount of the blast-furnace top gas that occurs in theproduction of pig iron and a partial amount of the converter gas thatoccurs in the converter steel works is mixed with a partial amount ofthe coke-oven gas that occurs in the coke-oven plant; and the mixed gasis used as a useful gas.
 26. The method according to claim 25, wherein:blast-furnace top gas and converter gas are mixed; a syngas is producedfrom the mixed gas after a gas-conditioning operation; and conditionedcoke-oven gas is admixed with the syngas or the cleaned mixed gas beforethe further processing to form the syngas.
 27. The method according toclaim 26, wherein a syngas is produced from blast-furnace top gas aftera gas-conditioning operation; and wherein conditioned coke-oven gas isadditionally admixed with the syngas or the cleaned blast-furnace topgas before the further processing to form the syngas.
 28. The methodaccording to claim 26, wherein a syngas is produced from converter gasafter a gas-conditioning operation; and wherein conditioned coke-ovengas is additionally admixed with the syngas or the cleaned converter gasbefore the further processing to form the syngas.
 29. The methodaccording to claim 25, wherein the externally obtained electricity is atleast partially obtained from renewable energy.
 30. The method accordingto claim 25, wherein the determination of the process parameter dependson a function that includes the price for the externally obtainedelectricity and the costs for producing the power-generating plantelectricity as variables.
 31. The method according to claim 30, whereinthe power output of the power-generating plant is controlled between 20%and 100%, in dependence on the amount of useful gas fed to thepower-generating process.
 32. The method according to claim 24, whereina gas-turbine power-generating plant or a gas-turbine and steam-turbinepower-generating plant is used as the power-generating plant.
 33. Themethod according to claim 24, wherein the power output of the chemicalplant or of the biotechnological plant is controlled in dependence onthe amount of mixed gas fed to this plant.
 34. The method according toclaim 33, wherein the chemical plant has a plurality of small unitsarranged in parallel, which are individually switched on or offdepending on the available stream of useful gas.
 35. A method foroperating a plant complex comprising a chemical plant coupled to ametallurgical plant, the metallurgical plant comprising a blast furnacefor producing pig iron, a converter steel mill, and a coke-oven plant,the method comprising: feeding, as a useful gas, at least a partialamount of at least one of a blast-furnace top gas that occurs in theproduction of pig iron, a converter gas that occurs in the convertersteel mill, and a coke-oven gas that is drawn off from the coke-ovenplant, to the chemical plant for producing chemical products.
 36. Amethod for operating a plant complex comprising a biotechnological plantcoupled to a metallurgical plant, the metallurgical plant comprising atleast one blast furnace for producing pig iron, a converter steel mill,and a coke-oven plant, the method comprising: feeding a partial amountof at least one of a blast-furnace top gas that occurs in the productionof pig iron, a converter gas that occurs in the converter steel mill,and a coke-oven gas that is drawn off from the coke-oven plant to abiotechnological plant for biochemical processes.